diff --git a/extensions/compass/stylesheets/compass/typography/_vertical_rhythm.scss b/extensions/compass/stylesheets/compass/typography/_vertical_rhythm.scss
index 999d1fb..9661b1c 100644
--- a/extensions/compass/stylesheets/compass/typography/_vertical_rhythm.scss
+++ b/extensions/compass/stylesheets/compass/typography/_vertical_rhythm.scss
@@ -127,6 +127,11 @@ $base-half-leader: $base-leader / 2;
 
 // Calculate the minimum multiple of rhythm units needed to contain the font-size.
 @function lines-for-font-size($font-size) {
+  // Make sure we have a value to do math with
+  @if !$font-size {
+    $font-size: $base-font-size;
+  }
+
   $lines: if($round-to-nearest-half-line,
               ceil(2 * $font-size / $base-line-height) / 2,
               ceil($font-size / $base-line-height));